1. Field of the Invention
The invention pertains generally to the manufacture of lead-acid storage batteries and is more particularly directed to the provision of novel structures for battery plate grids and processes for making the same.
2. Description of the Prior Art
The lead acid storage battery, commonly termed a "secondary battery" because of its reversible charge-discharge cycle, is well known in the art. Normally such batteries are comprised of a plurality of "cells" in either series or parallel connection depending upon the capacity and voltage desired. It is the cell that is the basic unit of the battery and the component in which the electrochemical reaction producing the emf of the battery takes place. Each cell consists of a plurality of plates, usually odd in number, alternately spaced in opposite positive and negative polarities. All positive plates in a cell are electrically connected together as are all negative plates and each plate in an opposing pair is electrically insulated from the other by an electrolyte permeable separator.
The battery plates for a lead-acid storage cell of this type have generally in the past been formed of a cast lead alloy grid structure with an active material pasted thereon. The grid structure usually has a multiplicity of gratinglike apertures into which a paste of active material, conventionally containing some form of lead or lead oxide, is applied.
In this manner the usual grid structure provides a number of functions for the storage battery. First, it supports the active material and provides the structural strength of the plate. Additionally, the grid structure is utilized for the more important function of conducting electric current from the active material to an external electrode and for maintaining a somewhat uniform current collection from the active material.
These lead alloy grids are manufactured conventionally by casting molten lead alloy into grating shaped molds. The lead alloy grids have a number of disadvantages being formed in this manner.
The primary disadvantage of cast grids is one of weight. There is an excess of lead conductor over that necessary to provide the important function of current conduction away from the active material. This additional lead does not participate actively in the electrochemical process and contributes unnecessary mass to the structure. A battery with such excessive mass will have lower energy density or mass utilization than one which uses an optimum amount of lead.
There have been attempts in the prior art to provide grids with an improved mass utilization. For example, plating lead onto a lighter grid material has been tried. Also, a U.S. Pat. No. 3,944,432, issued to Brinkmann et al, discloses a storage battery plate having lead bars positioned within channels defined by a grid of plastic strips. Further, a U.S. Pat. No. 3,944,431, issued to Ikari et al, describes a battery plate grid comprising a plurality of longitudinally spaced wire elements, each having a lead or lead alloy conductor helically wrapped around it. Although these prior art plates are successful to some extent in improving the mass utilization of lead, they are relatively expensive and complicated to manufacture and thus not entirely commercially acceptable. The problem therefore remains substantially unresolved by the prior art.
The use of excess lead in conventional cast grids has been mainly to provide the function of structural support for the battery plate and active material. It would alleviate this problem if the lead could be used only as a conductor and not structurally. However it is common in the art to alloy the lead with a number of elements to increase its structural strength for the support function at the expense of electrical considerations and corrosion resistance.
Normally, the most common additional element is that of antimony which increases the structural integrity of the lead and facilitates the casting process. Other known lead alloys are those which employ calcium, arsenic, tellurium, tin, and cadmium in combination with basic lead. The more exotic of these alloys are difficult and expensive to provide while the most common, antimony, is objectionable because it decreases the corrosion resistance of the lead. Lead-antimony alloy also promotes decomposition of the electrolyte upon charging and is less conductive than is pure lead conductor. Thus, it would be advantageous to use pure lead for battery grids instead of relying on alloys to provide additional stiffness to the lead because of its use as a structural support.
A further disadvantage of alloyed cast grids is their manufacture by the casting process. It would be beneficial to provide an improved process that would replace conventional battery plate grid manufacturing techniques.
Another problem to be confronted with presently available cast grids is that of "sloughing" or shedding. This phenomenon occurs when the active material becomes soft and disassociates from the grid structure. The adherence by the active material to the grid structure would be greater and cause less sloughing if the grating openings were smaller and less open. However, such a change would exacerbate the already serious mass utilization problem and use even more lead. Sloughing cuts the efficiency of the battery plates and may short out the cell if enough material becomes disloged and accumulates at the bottom of the cell between plates. The cycle life of a battery could be significantly altered by controlling this effect.